1. Field of the Invention
This invention relates to a device for extracting only the data components from various kinds of information modulated by different modulating systems.
2. Description of Prior Art
There are various kinds of magnetic memory (or recording) devices such as, for example, magnetic tape, magnetic disc, magnetic drum, etc., which are used for electronic computers. These magnetic memory devices are better than punched tape in view of their reliability and high processing speed. As one example, when a magnetic head 2 is contacted with a magnetic disc (recording medium) 1 as shown in FIG. 1 of the accompanying drawing and the disc is rotated in the direction of an arrow 3, there is formed a track 1a, whereby recording and reproduction of information are effected on the track 1a. The information read out of the track 1a by the magnetic head 2 is subjected to frequency-modulation (FM) or modified frequency-modulation (MFM) to be described later.
Signal waveforms concerning the frequency-modulation are shown in FIGS. 2A and 2B of the accompanying drawing, and signal waveforms concerning the modified frequency-modulation are shown in FIGS. 2C and 2D of the accompanying drawing. FIG. 2A shows the original signal, in which data consits of "1" and "0". FIG. 2B shows the frequency-modulated waveform of the original signal shown in FIG. 2A, in which the abovementioned data pulses are inserted between the adjacent clock pulses P.sub.C1, P.sub.C2, P.sub.C3, . . . . In other words, a pulse P.sub.D1 representing the data "1" is generated between the clock pulses P.sub.C1 and P.sub.C2, and a "no pulse" state representing the data "0" is generated between the clock pulses P.sub.C2 and P.sub.C3.
In such a frequency-modulating system, since the data pulse is present between the adjacent clock pulses, the distinction between the clock pulse and the data pulse is clear. However the signal density is disadvantageously low. A modulating system that remedies this shortcoming of such a frequency-modulating system, is the modified frequency-modulating system. FIG. 2C of the accompanying drawing shows the original signal similar to that shown in FIG. 2A, in which the data consist of "1", "0", "0", "1". FIG. 2D shows the waveform of the original signal shown in FIG. 2C which has been subjected to the modified frequency-modulation. In this waveform diagram, a clock pulse P.sub.C is provided at the intermediate section of the adjacent data "0", "0" in place of the abovementioned clock pulse shown in FIG. 2B. In other words, the pulses P.sub.D1 and P.sub.D2 in FIG. 2D represent the data "1" shown in FIG. 2C, and a pulse P.sub.C1 represents the clock pulse provided at the intermediate portion of the two data "0", "0". Thus, according to the modified frequency-modulation system, the quantity of the information is twice as dense as in the abovementioned frequency-modulation system (which will be seen upon comparison of FIGS. 2D and 2B) despite the fact that the pulse number per unit time is less. However, even in the modified frequency-modulation system, there is a disadvantage in that, when rotational irregularity occurs in the disc shown in FIG. 1, the system tends to be more readily affected than the frequency-modulation system with the consequence that errors in reading the data can more easily occur.
In view of the abovementioned advantages and disadvantages of the frequency-modulation system and the modified frequency-modulation system, both of them have so far been adopted equally in various kinds of computers depending on the type or model of such computers. Considering, however, the general use of the magnetic disc, it is desirable that either magnetic discs with information recorded under the frequency-modulation system or that recorded under the modified frequency-modulation system be used with the computer in common.